For a sustained infection, enteric bacterial pathogens must evade, resist or tolerate a variety of antimicrobial host defence peptides and proteins. We report here that specific organic acids protect stationary‐phase Escherichia coli and Salmonella cells from killing by a potent antimicrobial peptide derived from the human bactericidal/permeability‐increasing protein (BPI). BPI‐derived peptide P2 rapidly halted oxygen consumption by stationary‐phase cells preincubated with glucose, pyruvate or malate and caused a 109‐fold drop in cell viability within 90 min of addition. In marked contrast, O2 consumption and viability were not significantly affected in stationary‐phase cells preincubated with formate or succinate. Experiments with fdhH, fdoG, fdnG, selC and sdhO mutants indicate that protection by formate and succinate requires their oxidation by the Fdh‐N formate dehydrogenase and succinate dehydrogenase respectively. Protection was also dependent on the BipA GTPase but did not require the RpoS sigma factor. We conclude that the primary lesion caused by this cationic peptide is not gross permeabilization of the bacterial cytoplasmic membrane but may involve specific disruption of the respiratory chain. Because P2 shares sequence similarity with a range of other antimicrobial peptides, its cytotoxic mechanism has broader significance. Additionally, protective quantities of formate are secreted by E. coli and Salmonella during growth suggesting that such compounds are important determinants of bacterial survival in the host.

Pseudomonas fluorescens B52 produces substantial biofilms at the air/liquid/solid interface of glass coverslips clamped vertically and partly submerged in liquid medium at 21 degrees C. Biofilm formation was maximal ca. 20-50 h after inoculation of the liquid medium and as indicated by environmental scanning electron microscopy (ESEM), contained large numbers of bacterial cells that were embedded within an extensive exopolymeric matrix. Incubation beyond 50 h led to reductions in biofilm which ESEM related primarily to losses of exopolymer. Both biofilm formation and the subsequent decline in exopolymer deposition was more rapid, and occurred to greater extents, when supernatants from two-day old cultures of B52 were used as the initial growth media. The addition of N-acyl-hexanoyl homoserine lactone to fresh growth medium had a similar effect upon biofilm formation as using spent culture medium. Homoserine lactones could not be demonstrated in spent culture supernatants by an Agrobacterium tumefaciens bioassay. An exopolysaccharide lyase was detected in spend culture media taken from dense biofilm cultures whose action was specifically directed towards biofilm exopolysaccharide. Results suggest that (i) cell-cell signals such as homoserine lactones are associated with the formation of P. fluorescens biofilms, (ii) the enzymic degradation of exopolymers has a specific role in the detachment of cells under starvation conditions, and (iii) whilst short chain (C6) exogenous homoserines can trigger such response in P. fluorescens, its own signal substance is likely to possess a longer (> C8) fatty acyl chain.

A group of 80 Pseudomonas spp. strains isolated from raw milk shortly after milking was compared to another group of 81 obtained from the same sample after incubating it at 7°C for 3 days. Comparison of both collections of strains included growth rates at 7°C and 21°C and production of extracellular proteinase, lipase, and siderophores. The strains selected after cold incubation showed an average to-fold higher growth rate at 7°C, 1,000-fold more proteolytic activity, and 280-fold more lipolytic activity than those found before the incubation. At 21°C, however, they grew half as quickly as the strains isolated before the incubation. In all but one of the 161 Pseudomonas strains tested, there was some production of siderophores, and yields were only moderately increased in the strains obtained after incubation of the milk at 7°C. These changes in spoilage-related properties took place while global Pseudomonas counts increased only 13-fold. Distribution frequencies of the variables tested, their correlation coefficients, and regression models are shown.

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